The indirect fluorescent antibody test and a microhemagglutination technique detected antibodies in human sera to Legionnaires' disease antigen about equally. Since the hemagglutination technique was simpler, more rapid, and less expensive, we used it to ascertain the prevalence of antibody in 1200 sera from apparently healthy, nonhospitalized Michigan residents. Prevalence was analyzed by age, sex, geographic location (county of residence), and season of the year. There was a significant difference in prevalence between seasons: 91 of 600 sera (15.2%) from February to March 1978 had a titer equal to or greater than 1:16, contrasted with 179 of 600 sera (29.8%) from August to September 1978. This difference was independent of age and sex. There was no significant difference by geographic location, sex, or age except for decreased prevalence for persons 60 years or older.